Car engines are machines, and machines experience the occasional flaw. At some point, all car owners will inevitably have some form of engine trouble. Think of how much easier life would be if you could always diagnose and cure the issue with minimal wallet wear ‘n’ tear. Thanks to Tracy Martin’s How to Use Automotive Diagnostic Scanners, we can all hone our mechanical skills and save some real money on labor costs. There are many factors to be considered, one of which being the status of the engines fuel. The following excerpt discusses the importance and process in the use of the controller area network.
A CONTROLLER AREA NETWORK—CAN controller area network consists of multiple system components that use micro-controllers to communicate with each other. These communication systems are similar to integrated cable networks found in business offices, where desktop computers, file servers, printers, and digital phone systems all connect to, and communicate with, each other. However, controller area networks predate many office systems, as they were found as early as 1992 on some Mercedes Benz models, and in 1993 on BMW’s 740i/iL.
Starting in 2003, CAN was used on some OBD-II vehicles to communicate with scan tools, and in 2004, Ford, Mazda, Mercedes, and Toyota equipped all of their OBD-II vehicles exclusively with CAN for all vehicle-to-scan-tool communications. Developed by Intel, Bosch, and a few others, CAN communication technology has been around for over 20 years. In 2012 Bosch released CAN FD 1.0 or CAN Flexible Data-Rate. This specification uses a different frame format that allows a different data length as well as optionally switching to a faster bit rate. CAN FD is compatible with existing CAN 2.0 (first published in 1991) networks allowing CAN FD devices to coexist on the same network with existing CAN protocols.
The concept of how CAN works is simple: Computers, their sensors, and all power-consuming components on
a vehicle are connected to each other via a single wire; this single wire is really two wires twisted together and is called a “twisted pair.” The twisted pair of wires used by a CAN system to communicate within the computer system are called a BUS, which is, quite simply, nothing more than a glorified communication network. The BUS allows all electronic information to be available at all times within the network of components, since digital messages are sent out by each computer, or controller, and received by all computers connected to the network.
For example, assume a driver wants to turn on a vehicle’s high beams. A conventional automotive electrical system would operate as follows: The driver would pull back on the turn signal lever to switch on the high beams. This action would electrically trigger a relay that would, in turn, send power to the left and right high beam headlights. Performing this same function on a CAN type of system actually happens quite differently, and with much more sophistication. When the driver switches on the high beams, the chassis control computer receives the switched input and then triggers an internal power transistor (instead of a relay) that sends 12 volts to the high beams. But that’s not all.
In the event one of the high beam bulbs has burned out, the chassis controller would measure the current flowing to both lights (in this case the current to the high beams would be half of its normal rate), and would then send out a message that one of the high beams was not working via a unique identifier code along the BUS wire. All controllers or computers in the system would get the message and check to see if the message ID code applied to them (and the systems they specifically control). If the ID code did not apply to a particular computer or controller, the message would be ignored. However, when an instrument cluster controller receives the coded message, it recognizes the unique coded message as one that pertains to a system within its control, so it turns on a warning light on the instrument panel, informing the driver that there is a lighting system malfunction. In addition, an engine management controller also recognizes the message ID (i.e. one of the high beams is fried), and it also stores a trouble code in its memory for later retrieval by a technician using a scan tool connected to a diagnostic gateway.
Furthermore, significantly greater levels of diagnostics are available with CAN communication systems. For
example, if a high beam circuit had a wire that was shorted to ground, a CAN system would quickly measure the
current going to the high beam circuit, determine it is too high, and immediately shut off the power transistor that
supplies power to the high beams before any damage could occur. The action would prevent the chassis controller from moving into meltdown mode. In such a case, a different diagnostic message would be sent out across the network since the code stored in the engine management computer might read “Excessive Current in High Beam Circuit.” As a direct result of this superior level of circuit protection, no fuses would have to be used in the entire
automobile. In addition, controllers would serve to switch power on and off to various components via transistors, instead of mechanical relays. Thus, the only relay used on an entire vehicle would be a starter solenoid.
While these systems are capable of performing self-diagnosis on many electrical-related problems, at least to some extent, they must still utilize wiring harnesses (though smaller) and connectors, all of which frequently have the same common electrical problems technicians have come to know and love. Consequently, while the diagnostic capabilities a CAN system assists knowledgeable technicians with solutions to electrical problems, they will not
actually replace repair technicians any time in the near future.
Another example of how a CAN-BUS system integrates functions from multiple controllers can be seen in an alarm system. If an alarm is set, and a would-be thief disturbs the car, several messages are sent out by the alarm controller: The first message instructs the chassis controller to turn on the hazard lights and sound the horn. A second message is sent to the engine management controller which prevents operation of the electronic fuel pump. Once these messages are sent, no amount of “hot wiring,” ignition lock drilling, etc., will allow the engine to start or run.
Messages sent by controllers on some CAN systems are eight bytes long and travel at a speed of 500 k/bps (kilobytes per second) through the CAN-BUS system. This works out to roughly 4,500 messages per second. In fact, a CAN system transmits data at a rate of one megabyte per second, or 30 pages of information each second. If two messages are sent at the same time, they are prioritized; one will be sent immediately and the other will be stored in the memory of the sending controller until BUS traffic allows it to be sent. Using a voltmeter to monitor CAN messages does not work, as it will only show 2.5 volts on the BUS wires when a message is present.
Thus, a digital lab scope is the only way to actually see a message being sent. Another significant advantage of CAN systems over conventional automotive wiring is that fewer wires are used. For instance, the single wire BUS used on a CAN system utilizes shorter wires, which makes for a lighter wiring harness with more
reliability. Additional benefits include less interference with low-voltage electronic fuel injection sensor signals, enhanced diagnostic functions for the entire electrical system, and available software updates whenever new electrical components are added.
By 2008, all vehicles were required to use some form of CAN in order to ensure standardized communication with scan tools and other diagnostic equipment. Eventually, entire electrical systems on cars and trucks will use CAN—it’s only a matter of time.
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How to Use Automotive Diagnostic Scanners teaches you how to choose the right scanner for your application and how to use it, with a comprehensive list of what each code means. Photos and diagrams help you understand OBD-I and OBD-II systems (including CAN) and the scanners that read the information they record. From catalytic converters and O2 sensors to emissions and automotive detective work, this is the complete reference for keeping your vehicle EPA-compliant and on the road!
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